The exact evolutionary stage of the companion star in Cen X-3 is not well-known, as the measures of its spectral type range from O9 III-V [1]} to O6-7 II-III [2]}. If the donor is already set towards the giant phase, then the system could not have formed much earlier than 17 Myr ago in the case no mass was gained by the secondary before the HMXB phase.
